(a) Field of the Invention
The present invention relates to a method for preparing a highly pure 1,1,2,2-tetrakis(4-hydroxy-3,5-dimethylphenyl) ethane, an epoxy resin composition comprising, as a hardener, the highly pure compound prepared by the method, an epoxy resin derived from the highly pure compound prepared by the method, and methods for preparing the epoxy resin composition and the epoxy resin.
(b) Description of the Prior Art
1,1,2,2-tetrakis(4-hydroxy-3,5-dimethylphenyl)ethane (hereinafter referred to as "tetrakisxylenolethane" or "TKXE") has been used as, for instance, a starting material for preparing phenol resins or a hardener for epoxy resins
For instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 64-74213 discloses an epoxy resin composition comprising an epoxy compound obtained by epoxidating TKXE and TKXE, as a hardener in a specific mixing ratio in which these TKXE's are obtained by condensing 2,6-xylenol and glyoxal in the presence of an oxidizing catalyst.
Glyoxal is quite unstable in the anhydrous state and is in general put on the market in the form of an aqueous solution having a concentration ranging from 40 to 50% by weight. If such an aqueous solution of glyoxal is used as glyoxal component, the condensation of 2,6-xylenol and glyoxal is necessarily performed in the presence of water. When 2,6-xylenol and glyoxal are condensed in the presence of water, low molecular weight by-products such as glyoxal condensates are simultaneously produced in a large amount in addition to TKXE and these low molecular weight substances are very difficult to separate from TKXE. For this reason, the foregoing method cannot provide highly pure TKXE.
Accordingly, TKXE prepared by condensing 2,6-xylenol and glyoxal in the presence of an oxidizing catalyst, which contains impurities such as low molecular weight substances and which has a low purity, must be used in such a method as a starting material for preparing an epoxy resin or a hardener for epoxy resins.
If TKXE which contains low molecular weight substances such as glyoxal condensates is used as a starting material for preparing an epoxy resin or a hardener for epoxy resins, the resulting hardened product has low physical properties such as heat resistance and mechanical strength. Therefore, a problem arises when the hardened product is used in particular as a material for electric and/or electronic applications such as that for sealing IC's.
Moreover, J.P. KOKAI No. Sho 63-223020 discloses a method for preparing polyphenols which comprises condensing phenols with dialdehydes such as glyoxal and glutaraldehyde in the presence of an acid catalyst wherein the content of water in the reaction system is controlled to a level of not more than 2% by weight.
However, this method still uses dialdehydes in the form of aqueous solutions as in the foregoing methods although the water content of the reaction system is reduced to a level of not more than 2% by weight. Thus, in this method, it is required that a large amount of water contained in the reaction system is removed through, for instance, distillation prior to the condensation reaction to control the water content of the reaction system to not more than 2% by weight. These operations make the condensation reaction process complicated and require the use of excess energy for the removal of a large amount of water through, for instance, distillation. Moreover, the water removed from the reaction system contains a large amount of organic substances and, therefore, cannot be discharged out of the plant without any pretreatment. For this reason, it is required for the use of a water purifying plant such as an activated sludge processing apparatus. Furthermore, the formation of low molecular weight substances such as glyoxal condensates cannot completely be suppressed since the condensation reaction of phenols and dialdehydes is still performed in the presence of water although the content thereof is low of the order of not more than 2% by weight. Thus, this method has still been insufficient for use in the preparation of highly pure TKXE.